Core Capabilities & Approach: Complexes will be crystallized by vapor diffusion, microbatch, and microdialysis methods. After identification of initial crystallization conditions, optimization will be carried out to obtain high-quality crystals for X-ray diffraction. All crystals will be screened for diffraction quality at the home source; should diffraction be obtained that is not of sufficiently high resolution and/or quality, changes at the protein level will be carried out in the Protein Core. Such an iterative cycle wdll be continued until sufficiently high-quality diffracting crystals become available. Other possible avenues for obtaining better diffracting crystals include i) Identifying precipitant and protein concentrations that minimize showering of small crystals or excessive nucleation; 2) Identifying additives that improve resolution and expanding on these classes of chemicals; and 3) using annealing procedures to lower mosaicity and increase resolution. Crystallographic data will be mainly collected at the SERCAT facility sector 22-BM beam line of the Advance Photon Source at Argonne National Laboratory, Chicago, IL, permitting collection of high-resolution diffraction data on native protein complexes and multiple wavelength anomalous diffraction using Se-Met derivative crystals. The selenium atom sites and MAD phases will be automatically determined using the AutoSol module and initial model building with additional iterative density improvement will be done using the Autobuild module in the program Phenix(254-256)^ Xhe initial MAD structure will be used as a Molecular Replacement (MR) model and structures will be further refined through cycles of rebuilding and refinement using the program Coot(257, 258) and RefMac(259) a refinement program within the CCP4 package if necessary, we will carry out heavy atom derivatives to solve the crystallographic phase problem. iii. Proiect Component: In addition to screening activities, the Core wdll determine the structures of complexes containing DCAFi, DDBi, and Vpr/Vpx with associated substrates (for example, SAMHDi), other E3 ligase complexes, capsid related complexes, and Trimsa SPRY domain.